1. Field of the Invention
The present invention relates to motor speed controls and more specifically it relates to a motor speed control utilizing phase-lock loop techniques with tachometer feedback.
2. Description of the Prior Art
In many systems utilizing electric or hydraulic motors, to obtain variable speed rotary motion, feedback controls stabilize motor drive speeds that are subject to variations in loading and supply voltages or pressure. To minimize the difference between the desired speed and the actual speed of the feedback control motor drive, the open loop gain of the system must be high. Many feedback control systems integrate the loop error signal to obtain a loop control signal that produces a steady state or dc gain that approaches infinity and a loop error that approaches zero.
The practical implementation of such feedback control usually involves a tachometer, such as for example, that formed by a variable reluctance magnetic pick-up in conjunction with a gear wheel of ferrous metal, to sensor motor speed, and operational amplifiers to sum, integrate, and scale signals. The magnetic pick-up produces a signal that is approximately sinusoidal, having amplitude and frequency that are related to the motor speed. This signal, used in a conventional control loop for hydraulic motors, must be converted to a dc signal either by peak rectification or by frequency-to-dc voltage conversion, adding circuit complexity and cost, while degrading accuracy and reliability. Additionally, practical operational amplifiers used as integrators have finite dc gain, thus loop errors can not be totally eliminated. Several limitations of the conventional feedback controller may be avoided with the utilization of a phaselock loop to implement the integration function.
Systems utilizing phaselock loops for electric motor control are disclosed in U.S. Pat. Nos. 3,176,208 and 3,753,067. The error signal used for the motor control in these systems is related to the difference in phase between a reference waveform and the waveform at the output terminals of a tachometer coupled to the motor shaft. Phase locked loops in these systems act as ideal integrators, and provide a control that reduces the speed error substantially to zero.
These systems are for the control of electric motors, a task much more simple than the control of hydraulic drives. Electric motors are adequately characterized for speed and torque performance by simple linear equations and generally do not contribute significant time constants to a mechanical control loop. By contrast hydraulic drives are characterized by non-linear equations, due to the square law relationship of flow to pressure, and due to fluid compressibility almost always contribute significant time constants to the control loop. This presents special control loop stability problems that are especially troublesome in phase locked systems.
Each phase locked loop is characterized by a center frequency, a lock-in range, and a tracking range. A properly designed phase locked control must insure that the motor speed increases after starting to a speed that produces a tachometer output signal frequency within the lock-in range of the loop. Additionally, it must insure that after lock-in, transients do not cause the loop to lose lock. It is possible, in poorly designed systems, that lock will not be acquired initially or re-established after a transient disturbance.